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1. ALMA-IMF: IV. A comparative study of the main hot cores in W43-MM1: Detection, temperature, and molecular composition

   https://doi.org/10.1051/0004-6361/202243669  

   Brouillet, N; Despois, D; Molet, J; Nony, T; Motte, F; Gusdorf, A; Louvet, F; Bontemps, S; Herpin, F; Bonfand, M; et al (September 2022, Astronomy & Astrophysics)

   Context.Hot cores are signposts of the protostellar activity of dense cores in star-forming regions. W43-MM1 is a young region that is very rich in terms of high-mass star formation, which is highlighted by the presence of large numbers of high-mass cores and outflows. Aims.We aim to systematically identify the massive cores in W43-MM1 that contain a hot core and compare their molecular composition. Methods.We used Atacama Large Millimeter/sub-millimeter Array (ALMA) high-spatial resolution (~2500 au) data to identify line-rich protostellar cores and carried out a comparative study of their temperature and molecular composition. Here, the identification of hot cores is based on both the spatial distribution of the complex organic molecules and the contribution of molecular lines relative to the continuum intensity. We rely on the analysis of CH₃CN and CH₃CCH to estimate the temperatures of the selected cores. Finally, we rescale the spectra of the different hot cores based on their CH₃OCHO line intensities to directly compare the detections and line intensities of the other species. Results.W43-MM1 turns out to be a region that is rich in massive hot cores. It contains at least one less massive (core #11, 2M_⊙) and seven massive (16−100M_⊙) hot cores. The excitation temperature of CH₃CN, whose emission is centred on the cores, is of the same order for all of them (120–160 K). There is a factor of up to 30 difference in the intensity of the lines of complex organic molecules (COMs). However the molecular emission of the hot cores appears to be the same or within a factor of 2–3. This suggests that these massive cores, which span about an order of magnitude in core mass, have a similar chemical composition and show similar excitation of most of the COMs. In contrast, CH₃CCH emission is found to preferentially trace the envelope, with a temperature ranging from 50 K to 90 K. Lines in core #11 are less optically thick, which makes them proportionally more intense compared to the continuum than lines observed in the more massive hot cores. Core #1, the most massive hot core of W43-MM1, shows a richer line spectrum than the other cores in our sample, in particular in N-bearing molecules and ethylene glycol lines. In core #2, the emission of O-bearing molecules, such as OCS, CH₃OCHO, and CH₃OH, does not peak at the dust continuum core centre; the blueshifted and redshifted emission corresponds to the outflow lobes, suggesting formation via sublimation of the ice mantles through shocks or UV irradiation on the walls of the cavity. These data establish a benchmark for the study of other massive star-formation regions and hot cores. 

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2. ALMA-IMF: XI. The sample of hot core candidates: A rich population of young high-mass protostars unveiled by the emission of methyl formate

   https://doi.org/10.1051/0004-6361/202347856  

   Bonfand, M; Csengeri, T; Bontemps, S; Brouillet, N; Motte, F; Louvet, F; Ginsburg, A; Cunningham, N; Galván-Madrid, R; Herpin, F; et al (July 2024, Astronomy & Astrophysics)

   Context.The star formation process leads to an increased chemical complexity in the interstellar medium. Sites associated with high-mass star and cluster formation exhibit a so-called hot core phase, characterized by high temperatures and column densities of complex organic molecules. Aims.We aim to systematically search for and identify a sample of hot cores toward the 15 Galactic protoclusters of the ALMA-IMF Large Program and investigate their statistical properties. Methods.We built a comprehensive census of hot core candidates toward the ALMA-IMF protoclusters based on the detection of two CH₃OCHO emission lines at 216.1 GHz. We used the source extraction algorithm GExt2D to identify peaks of methyl formate (CH₃OCHO) emission, a complex species commonly observed toward sites of star formation. We performed a cross-matching with the catalog of thermal dust continuum sources from the ALMA-IMF 1.3 mm continuum data to infer their physical properties. Results.We built a catalog of 76 hot core candidates with masses ranging from ~0.2M_⊙to ~80M_⊙, of which 56 are new detections. A large majority of these objects, identified from methyl formate emission, are compact and rather circular, with deconvolved full width at half maximum (FWHM) sizes of ~2300 au on average. The central sources of two target fields show more extended, but still rather circular, methyl formate emission with deconvolved FWHM sizes of ~6700 au and 13 400 au. About 30% of our sample of methyl formate sources have core masses above 8M_⊙and range in size from ~1000 au to 13 400 au, which is in line with measurements of archetypical hot cores. The origin of the CH₃OCHO emission toward the lower-mass cores may be explained as a mixture of contributions from shocks or may correspond to objects in a more evolved state (i.e., beyond the hot core stage). We find that the fraction of hot core candidates increases with the core mass, suggesting that the brightest dust cores are all in the hot core phase. Conclusions.Our results suggest that most of these compact methyl formate sources are readily explained by simple symmetric models, while collective effects from radiative heating and shocks from compact protoclusters are needed to explain the observed extended CH₃OCHO emission. The large fraction of hot core candidates toward the most massive cores suggests that they rapidly enter the hot core phase and that feedback effects from the forming protostar(s) impact their environment on short timescales. 

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3. ALMA Observations of Massive Clouds in the Central Molecular Zone: External-pressure-confined Dense Cores and Salpeter-like Core Mass Functions

   https://doi.org/10.3847/1538-4357/ad9f28  

   Zhang_张, Zhenying 朕荧; Lu_吕, Xing 行; Liu_刘, Tie 铁; Qin_秦, Sheng-Li 胜利; Ginsburg, Adam; Cheng_程, Yu 宇; Liu, Hauyu Baobab; Walker, Daniel L; Tang_汤, Xindi 新弟; Li_李, Shanghuo 尚活; et al (February 2025, The Astrophysical Journal)

   We present Atacama Large Millimeter/submillimeter Array Band 6 (1.3 mm) observations of dense cores in three massive molecular clouds within the central molecular zone (CMZ) of the Milky Way, including the Dust Ridge cloud e, Sgr C, and the 20 km s⁻¹cloud, at a spatial resolution of 2000 au. Among the 834 cores identified from the 1.3 mm continuum, we constrain temperatures and linewidths of 253 cores using local thermodynamic equilibrium methods to fit the H₂CO and/or CH₃CN spectra. We determine their masses using the 1.3 mm dust continuum and derived temperatures, and then evaluate their virial parameters using the H₂CO and/or CH₃CN linewidths and construct the core mass functions (CMFs). We find that the contribution of external pressure is crucial for the virial equilibrium of the dense cores in the three clouds, which contrasts with the environment in the Galactic disk where dense cores are already bound, even without the contribution of external pressure. With our new temperature estimates we also find that the CMFs show a Salpeter-like slope in the high-mass (≳3–6M_⊙) end, a change from previous works. Combined with the possible top-heavy initial mass functions (IMFs) in the CMZ, our result suggests that gas accretion and further fragmentation may play important roles in transforming the CMF to the IMF. 

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4. Chemical Differentiation around Five Massive Protostars Revealed by ALMA: Carbon-chain Species and Oxygen/Nitrogen-bearing Complex Organic Molecules

   https://doi.org/10.3847/1538-4365/acd110  

   Taniguchi, Kotomi; Majumdar, Liton; Caselli, Paola; Takakuwa, Shigehisa; Hsieh, Tien-Hao; Saito, Masao; Li, Zhi-Yun; Dobashi, Kazuhito; Shimoikura, Tomomi; Nakamura, Fumitaka; et al (June 2023, The Astrophysical Journal Supplement Series)

   Abstract We present Atacama Large Millimeter/submillimeter Array Band 3 data toward five massive young stellar objects (MYSOs), and investigate relationships between unsaturated carbon-chain species and saturated complex organic molecules (COMs). An HC 5 N ( J = 35–34) line has been detected from three MYSOs, where nitrogen (N)-bearing COMs (CH 2 CHCN and CH 3 CH 2 CN) have been detected. The HC 5 N spatial distributions show compact features and match with a methanol (CH 3 OH) line with an upper-state energy around 300 K, which should trace hot cores. The hot regions are more extended around the MYSOs where N-bearing COMs and HC 5 N have been detected compared to two MYSOs without these molecular lines, while there are no clear differences in the bolometric luminosity and temperature. We run chemical simulations of hot-core models with a warm-up stage, and compare with the observational results. The observed abundances of HC 5 N and COMs show good agreements with the model at the hot-core stage with temperatures above 160 K. These results indicate that carbon-chain chemistry around the MYSOs cannot be reproduced by warm carbon-chain chemistry, and a new type of carbon-chain chemistry occurs in hot regions around MYSOs. 

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5. The Initial Conditions of Clustered Core Collapse: Multiwavelength Analysis of Oph A SM1N and N6 at 100 au Resolution

   https://doi.org/10.3847/1538-4357/ad2857  

   Friesen, Rachel K; Bourke, Tyler L; Caselli, Paola; Di_Francesco, James; Li, Zhi-Yun; Pineda, Jaime E (April 2024, The Astrophysical Journal)

   Abstract We present new Atacama Large Millimeter/submillimeter Array (ALMA) continuum and NH₂D, N₂D⁺, and H₂D⁺line emission at matched, ∼100 au resolution toward the dense star-forming cores SM1N and N6 within the Ophiuchus molecular cloud. We determine the density and temperature structure of SM1N based on radiative transfer modeling and simulated observations of the multiwavelength continuum emission at 0.8, 2, and 3 mm. We show that SM1N is best fit by either a broken power-law or Plummer-like density profile with high central densities (n∼ 10⁸cm⁻³), and an inner transition radius of only ∼80–300 au. The free-fall time of the inner region is only a few ×10³yr. The continuum modeling rules out the presence of an embedded first hydrostatic core (FHSC) or protostar. SM1N is therefore a dynamically unstable but still starless core. We find that NH₂D is likely depleted at high densities within SM1N. The nonthermal velocity dispersions increase from NH₂D to N₂H⁺and H₂D⁺, possibly tracing increasing (but still subsonic) infall speeds at higher densities as predicted by some models of starless core contraction. Toward N6, we confirm the previous ALMA detection of a faint, embedded point source (N6-mm) in 0.8 mm continuum emission. NH₂D and N₂D⁺avoid N6-mm within ∼100 au, while H₂D⁺is not strongly detected toward N6. The distribution of these tracers is consistent with heating by a young, warm object. N6-mm thus remains one of the best candidate FHSCs detected so far, although its observed (sub)millimeter luminosity remains below predictions for FHSCs. 

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